Use endpoint argument to avoid collision with flask-admin Network

If a selectfield is disabled, None is returned (not a string).
To pass wtforms validation, that value shall be part of the choices.

We use None as the value for SelectField that are optional (instead of
'').
We were previously converting '' to None when creating a model.
We now use the coerce function to let the form convert 'None' to None.

main blueprint splitted in:
- inventory
- networks

- refactor Network class

This can be done via the Admin interface but this latter shall be
restricted to admin users. This simple page allows the group "create"
to easily add new Manufacturer, Model or Location.